Regarding populations, there are historic ethnic or linguistic groups that could be taken into account, but I picture that more for the purpose of creating alliances. The USA/Canada would be a 'melting pot' where Japan would have problems convincing anyone they were related, especially considering the Japanese themselves had isolated themselves so effectively for so long. In fact, given the USA's sort of beacon for every tired, poor opportunist and refugee, it might be hard to find a logical partner outside of Canada for alliance. It would have to be worded like 'North American Protectorate' which could include everything up to panama and the Caribbean, or 'Manifest Destiny' tree which includes taking all the islands in the Pacific. So, here we go.
Central American Goals
New Aztlan (Aztec Empire from the Rio Grande to Panama with cores on Southwestern USA states)
Communist Paradise (Trotsky)
Montezuma's Revenge (as Mexico, conquer Caribbean, then Spain)
South American Goals
Native Revolution (Indigenous rising!)
Catholic Paradise (Ally or conquer all of South America, also available for Spain, Portugal, eventual war over remaining French/Dutch/English colonies on north coast)
New World Economic Revolution (Economic war with North America, goal to create an economy as robust as North America's)
Japanese Goals
Create, peacefully if possible, the Co-prosperity sphere.
Island Empire Peacefully Annex all the islands in the pacific, if possible, via coups and economic pressure/coersion, helping in wars for 'independance.'
Empire of the Sun: Convert East Asia to bow down to Hirohito, through force if necessary.
Chinese Goals
Unify China
Island Empire (Rebuild and conquer all of the Pacific)
Economic Dominance (play the long game of becoming the economic powerhouse of Asia, dominating trade throughout the eastern hemisphere)
Maoist Revolution
India/Penninsula goals
Independance and alliance from Europe (Raj, Siam, Vietnam, Pakistan, Cambodia, Laos, Burma)
Hindu Empire (convert all governments in SE asia to the same government and allied)
Breathing Room. Conquer Australia, cuz there's too many people in SE Asia, and Australia looks inviting.
Australia/New Zealand
New Commonwealth. Ally with each other, puppet or annex (depending on government type) all of the pacific islands with Japan as the prize.
Indonesia
Islanders Rise! Break free from Europe, uniting all the island populations into a coalition and defend the islands from all external powers.
South Africa
Take over all of Africa, first by supporting revolutions in Africa, then incorporating them into your empire.
Homecoming. Conquer the UK and Netherlands, moving the capital to Europe, and puppeting South Africa (or releasing it)
Africa
New Egypt. Break away from the UK, create a new empire spanning all of once colonial Africa, through supporting revolutions and allying or conquering the new states.
Liber-de-Africa As Liberia, support revolutions and coups all over Africa, the goal to remove European influence, South Africa being the end prize.
Arabia/Middle East
Conquer all of the middle east, including either through diplomacy or war, kicking out all western powers. Prizes are Turkey and Egypt.
Crusader King. Starting with a free Lebanon or Israel, ally with a European power and facilitate the takeover of the middle east by that power, prizes being Ciaro, Turkey and Persia.
Russia
The Cold War (without declaring war on anyone, create an economy rival or better to that of the USA)
Revenge of the Romanovs (Rebuild the Russian Empire under a new Tsar)
Benevolent Communism (using politics, convert all warsaw pact nations to communism and ally with them)
Slavic Nations
Free the Slavs! Create a Slavic Alliance between Czechs and Pols, then free Belarus and Ukraine.
Balkan States
Unite the desparate: As Bulgaria, Greece, Romania, Yugoslavia, Albania, or Turkey unite or conquer the rest on the list. The prize is Italy.
Austria Hungary
Recreate the full Austro-Hungarian Empire without interference or help from the Allies, Axis, or Comintern. Then complete the Empire by taking Greece and Italy.
Scandinavia
Utopia of the North: Unite all of Scandinavia and Baltic States peacefully.
Viking Empire: Conquer all of Scandinavia and the Baltic States. The prize is Germany. (Secret goals: Sac Paris, Rome, London, and conquer Newfoundland)
Germany
Long live the Kaiser! Convert the country into a constitutional monarchy with the Kaiser in charge and restore borders. (stretch goals, get the colonies back, one way or another)
Switzerland
Create the European Union (Ally with all major powers in Europe, with focus tree that gives you mad political points)
France
Napoleon's Revenge! Conquer Moscow.
Free the minds of Europe! (convert all countries into a non-hostile type, ally with them)
England
English law makes the world English (Restore the King, annex all the colonies, then conquer the USA and bring it back under the crown)
Spain, Portugal
Restore the historic empires. Spain's stretch goal includes Florida, California.
Ireland
Irish Tea Party. Through influence, help commonwealth colonies rebel, then help Scotland and Wales rebel, then Annex England.
North America
North American Alliance (convert all of Western Hemisphere to democracy, then ally with them)
Manifest Destiny (For USA, Canada)
Segregated America ([weird, I know] Segregate the Northwest and southern Rockies for American Indians, Free states of California, Texas, Confederacy run by African people, New England and midwest for European people. This would have to be done by 'releasing' nations, I suppose. IDK, just brainstorming.)